Microwave oven and antenna structure therefor

ABSTRACT

The invention in different forms is shown as a serpentine antenna embedded in a microwave-transparent plate, nested in one surface of such a plate, and deposited on one surface of such a plate. A pair of such plate-antenna structures are mounted in the top and bottom of an oven cavity. The center of each antenna has an integral probe extending through openings in the back wall of the cavity and into the arms of a waveguide T. Energy from a magnetron is injected into the leg of the T, and divides between the arms to excite the antennas which then radiate microwave energy into the cavity. The material of the plates serves to absorb some microwave energy, whereby to present a desired minimum load to the magnetron when it is operating while the cavity is empty. The actual oven enclosure for cleaning purposes is that defined by the plates and the portions of the walls and door between the plates. These surfaces are flat, whereby they can quickly be cleaned with a minimum of effort.

United States Patent Inventor Edward A. White Fort Wayne, Ind.

Appl. NO. 804,176

Filed Mar. 4, 1969 Patented Feb. 9, 1971 Assignee Technology Instrument Corporation Newbury Park, Calif. a corporation of California MICROWAVE OVEN AND ANTENNA STRUCTURE THEREFOR 7 Claims, 4 Drawing Figs.

U.S. CI

Int. Cl

Field of Search References Cited UNITED STATES PATENTS 3,196,243 7/1965 Schall 3,316,380 4/1967 ransin g fijjj2:11:11:

3,320,396 5/1967 Boehm 219/1055 3,440,386 4/1969 Boehm 219/1055 Primary ExaminerJ. V. Truhe Assistant ExaminerL. H. Bender Attorney-Jerry E. Turner ABSTRACT: The invention in different forms is shown as a serpentine antenna embedded in a microwave-transparent plate, nested in one surface of such a plate, and deposited on one surface of such a plate. A pair of such plate-antenna structures are mounted in the top and bottom of an oven cavity. The center of each antenna has an integral probe extending through openings in the back wall of the cavity and into the arms of a waveguide T. Energy from a magnetron is injected into the leg of the T, and divides between the arms to excite the antennas which then radiate microwave energy into the cavity. The material of the plates serves to absorb some microwave ene gy, whereby to present a desired minimum load to the magnetron when it is operating while the cavity is empty. The actual oven enclosure for cleaning purposes is that defined by the plates and the portions of the walls and door between the plates. These surfaces are flat, whereby they can quickly be cleaned with a minimum of effort.

PATENTED FEB swan I TI 7',

MICROWAVE OVEN AND ANTENNA STRUCTURE THEREFOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to microwave ovens in which elements in the oven cavity serve to function as antennas for radiating microwave energy throughout the cavity, and which may also serve as thermal heating units.

2. Description of the Prior Art In US. Pat. No. 3,320,396 to Helmut Boehm, assigned to the same assignee as the present application, there is disclosed dual purpose heater elements in a microwave oven cavity. The elements are adapted to be excited by microwave energy from a magnetron, and function as traveling wave antennas to radiate microwave energy throughout the cavity. The elements are also adapted to be heated thermally for conventional browning, .broiling and the like.

For microwave cooking, the elements are spaced a predetermined distance from the adjacent top and bottom walls of the cavity, e.g., a quarter of a wavelength. For a magnetron operating in the vicinity of 2500 megacycles, for example, this physical spacing is quite small. Over a period of use, the elements become coated with materials driven off foodstuffs cooked in the cavity. Because of the close physical spacing just mentioned, it is somewhat difficult to thoroughly clean the elements. For this reason, it occasionally happens that a housewife does not expand the effort to do the necessary cleaning, and the buildup of materials on the elements becomes so great as to markedly lessen the desired cooking efficiency.

In addition, microwave ovens are occasionally used to cook only foodstuffs which constitute relatively light loads for the magnetron. For this reason, the magnetron is always operating in circumstances in which there is an undesirably large amount of energy reflection to the magnetron. No antenna arrangement presently known is effective to avoid the long term damaging effects on such magnetrons.

SUMMARY OF THE INVENTION My invention embraces an antenna element for microwave ovens embedded in or otherwise supported on a plate made of material that is capable of absorbing a desirable amount of such energy. Also embraced is an oven employing such plates to define opposed walls of an enclosure for foodstuffs to be cooked by microwave and/or thermal energy radiated by such elements. With this invention, I provide an oven enclosure which is characterized by maximum ease of cleaning, while at the same time providing a minimum constant load for the microwave energy source.

DESCRIPTION OF THE DRAWING FIG. I is a perspective view of a microwave-transparent plate in which an antenna element is embedded;

FIG. 2 is a fragmentary sectional view of a microwave oven employing a pair of such plates to form opposed walls of an enclosure for foodstuffs to be cooked in the oven;

FIG. 3 is an end view of another microwave-transparent plate in which the antenna element is nested or seated in one surface; and

FIG. 4 is an end view of a microwave-transparent plate wherein the antenna element is deposited on one surface.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Referring to FIG. 1, an antenna element shown in serpentine form is embedded in a plate 12. As shown at l4, 16, the ends of the antenna 10 extend through one edge of the plate 12. Also, a probe 18 hasan end portion embedded in the plate, and is connected to the antenna 10 intermediate its ends. In this particular example, the probe is connected to the center portion of the antenna.

Referring to FIG. 2, there is shown a microwave oven 20 of conventional construction, the door 22 thereof being illustrated in phantom lines. Also shown in phantom lines is a tray 24 for supportingfoodstuff 26 to be cooked in the oven. The tray is placed on a rack 28 carried on supports 30 which are secured in the side walls of the oven.

As shown, a pair of such plates I2, 12' are placed adjacent the top and bottom walls 32, 34, with the bottom plate below the rack 28. The probes 18 extend through the back wall 36 of the oven and into the arms 36, 38 of a waveguide tee 40. In the arrangement shown, the probes 18 extend through tubular elements 44 which are secured at one end in the back wall 36 of the oven, and at their other ends in the waveguide arms 36, 38.

The leg 46 of the tee extends from the center of the length of waveguide forming the arms 36, 38. Adjacent the other end of the leg 46 is a microwave energy source 48. The magnetron of such source has an output probe 50 and its surrounding shield 52 extending into the leg 46, whereby energy propagated down the leg 46 is divided between the arms 36, 38 to excite the probes l8 and thereby cause the antenna elements 10 to radiate microwave energy into the oven cavity.

My invention is not limited to the illustrated antenna shapes or particular means for exciting them with microwave energy. Further, the antennas may be elements that are adapted to be heated for cooking foodstufi's by conventional thermal energy, and to this end, the ends 14, 16 of the elements 10, which extend through openings in the back wall 36 of the cavity, may be connected to an external current source.

As indicated in FIG. 2, the plates 12,12 are attached to the surrounding vertical walls of the cavity so as to prevent materials from collecting between the walls and the edges of the plates. Thus, for the housewife, the oven enclosure is constituted of the confronting surfaces of the plates 12 and the surrounding vertical walls (flat or corrugated) between them. These are smooth surfaces which can be quickly and thoroughly wiped clean. The microwave cavity, however, is constituted of the entire enclosure, including the inner surfaces of the top and bottom walls 32, 34.

In this latter connection, while the plates I2, 12' readily permit microwave energy to pass through them, they nevertheless absorb some of the energy. Also, they are capable of withstanding the high temperatures encountered in such an oven, whether such heating is due to microwave energy or thermal heating. Materials suitable for these purposes include pyroceramics, Teflon, heat-treated glass and the like. Due to their energy absorption characteristics, the plates constitute a minimum load for the microwave energy source. Thus, a microwave oven in accordance with my invention is one in which the microwave energy source is never subjected to undesirably high energy reflection, as where the oven is used continuously for microwave cooking of foodstuffs that are characterized as extremely small loads.

Further in accordance with my invention, the oven 20 is designed to facilitate the escape of vapors while preventing the escape of microwave energy therefrom. To this end, the upper plate 12 is provided with a plurality of vertical openings 54, and sleeve inserts 56 extending through the upper wall 34 have their bores aligned with the openings 54. The dimensions of the aligned openings in the plate and the sleeves 56 are such as to prevent the undesired escape of microwave energy therethrough.

FIGS. 3 and 4 illustrate modified forms of antenna and supporting plates in accordance with my invention. In FIG. 3, a plate 60 has an antenna element 62 embedded in one surface, and in FIG. 4, an antenna element 64 is carried on one surface of a plate 66. These may be solid antenna elements, or they may be formed as printed elements, e.g., by silk screen or vacuum deposit processes.

The plates l2, I2, 60, 66 may be formed so as to absorb a controlled amount of microwave energy. The amount of absorption depends upon the ingredients and proportions thereof. For a plate of any given material, its energy absorption characteristics are directly related to its thickness.

Further, plates such as 60, 62, can be positioned in the oven so that the cooking effects vary in accordance with their antenna location. Thus, such plates can be positioned so their antenna elements face each other, or so that the antennas face the top and bottom walls 32, 34. Also, the substitution of plates 60, 62 for those shown in FIG; 2 results in an oven enclosure which can quickly be thoroughly cleaned.

I claim:

1. The combination, for a microwave oven,"of: a plate to be placed in the oven; and an antenna element integral with said plate; said antenna element having an integral portion adapted to couple energy thereto from an external source to cause said element to radiate microwave energy into the oven; and said plate being operable to permit radiation of energy from said element while absorbing a predetermined amount thereof, whereby to present a minimum load for the external source.

2. The combination of claim 1, wherein said element is embedded in said plate. Y

3. The combination of claim 1, wherein said element is on one surface of said plate.

4. The combination of claim 1, wherein said element is nested in one surface of 'said'plate. I

5. The'combination of claim '1, including: amicrowave'oven havinga metal cavity and a door through which to place food I stuff in the cavity; and 'a pair'of spaced plates as defined in claim 1 being positioned in said oven adjacent opposite walls of said cavity; said plates being arranged so as'to prevent materials-from'collecting between their edges and the sur- 7. The combination oi claim 6, -wherein the top-plate andthe top cavity wall have a plurality of aligned openings through which to permit vapors to escape from said cavity; said openings being dimensioned to prevent the escape of microwave energy from said cavity. 

1. The combination, for a microwave oven, of: a plate to be placed in the oven; and an antenna element integral with said plate; said antenna element having an integral portion adapted to couple energy thereto from an external source to cause said element to radiate microwave energy into the oven; and said plate being operable to permit radiation of energy from said element while absorbing a predetermined amount thereof, whereby to present a minimum load for the external source.
 2. The combination of claim 1, wherein said element is embedded in said plate.
 3. The combination of claim 1, wherein said element is on one surface of said plate.
 4. The combination of claim 1, wherein said element is nested in one surface of said plate.
 5. The combination of claim 1, including: a microwave oven having a metal cavity and a door through which to place foodstuff in the cavity; and a pair of spaced plates as defined in claim 1 being positioned in said oven adjacent opposite walls of said cavity; said plates being arranged so as to prevent materials from collecting between their edges and the surrounding walls of the cavity; and said plates forming opposed walls of an enclosure within the cavity for the foodstuff to be cooked.
 6. The combination of claim 5, wherein said plates are horizontal and are positioned adjacent the top and bottom walls of the cavity.
 7. The combination of claim 6, wherein the top plate and the top cavity wall have a plurality of aligned openings through which to permit vapors to escape from said cavity; said openings being dimensioned to prevent the escape of microwave energy from said cavity. 